1. Field of the Invention
Carbonylation of olefins, alcohols, acids and esters represents a very important, commercially developed processing scheme. In this process an olefin, an alcohol, an acid or an ester is contacted with carbon monoxide in the presence of a carbonylation catalyst. Many such catalysts have been disclosed and utilized in the prior art.
Recently, rhodium-containing compounds, preferably rhodium halides such as the iodide and the bromide, have been used in commercially established processes. The advantage of this class of catalysts over earlier catalytic agents utilized in this art is that the extreme thermodynamic conditions required in carbonylation reactions of the prior art are significantly eased. Specifically, the high carbon monoxide partial pressure required in reactions utilizing prior art carbonylation catalysts is decreased. Those skilled in the art are aware of the advantages obtained when the pressure of a reaction is decreased.
Although the utilization of these recently developed catalyst systems have significantly advanced the carbonylation art, this catalyst system is subject to an important failing. That failing relates to the fact that rhodium halides are homogeneous in the carbonylation reaction mixture and thus separation of the carbonylation product from the liquid reaction mixture containing the homogeneous catalyst is difficult. As a result, the removal of the product results in the removal of catalyst included therewith. In view of the high cost of catalyst, this failing results in significantly higher processing costs than would be the case if the catalyst could be separated from the product.
To overcome this critical deficiency proposals have been advanced wherein at least a portion of the liquid reaction mass is transferred from the reactor to a separation zone of substantially lower pressure. Therein, in the absence of heat, a portion of the carbonylation product is flashed. The unvaporized liquid carbonylation reaction mixture remaining, including the homogeneous catalyst, is recycled back into the reactor zone. With this scheme the advantages of a homogeneous catalyst system are obtained while overcoming its attendant separation problem.
While this advance in the art represents an improvement over prior art processes utilizing rhodium-containing catalysts, it still is subject to an important failing. That is, during the separation step the soluble rhodium-containing homogeneous catalyst is subject to precipitation. Those skilled in the art are aware of the criticality of this effect. Although the use of a processing scheme which includes a separation zone eliminates the loss of catalyst dissolved in the removed product, it substitutes a similar failing, the loss of high cost rhodium-containing catalyst by precipitation. An amount of soluble rhodium-containing compound catalyst, equal to that lost to precipitation in the separation zone, must thus be added to the unflashed liquid therein.
To overcome this serious deficiency a process has been developed wherein tin or a tin-containing compound is added to the carbonylation product mixture in the separation zone. The addition of tin or a tin compound has the effect of maintaining the rhodium compound catalyst in solution. Although the introduction of tin or a tin compound maintains the rhodium catalyst in solution, and thus eliminates the problem associated with rhodium precipitation, it creates still another problem. Those skilled in the art are aware that tin or a tin-containing compound is highly reactive. It reacts with the halogen component present in the catalyst to produce tin halide salts. These tin halide salts oftentimes evaporate under the aforementioned flashing conditions and are carried downstream where they contact metallic processing equipment. Since tin halide salts are corrosive, they create a major corrosion problem in the processing equipment utilized in the carbonylation reaction.
The above discussion not only emphasizes the desirability of developing a new processing scheme but also of substituting other equally effective carbonylation catalysts which provide the advantages obtained by the use of rhodium halides without the attendant problems discussed above.
2. Background of the Prior Art
The references relevant to the present invention include those disclosures which describe the developments discussed above. Thus, U.S. Pat. No. 3,579,552 to Craddock et al. discloses a process for the preparation of carboxylic acids by the reaction of ethylenically unsaturated compounds with carbon monoxide and water in the presence of catalyst systems which include rhodium compounds and complexes together with an iodide promoter.
A related development is described in U.S. Pat. No. 3,769,326 to Paulik et al. The '326 patent is similar to the teaching of the aforementioned '552 patent but is specifically directed to the reaction of an aromatic alcohol or ester, ether or halide derivatives thereof. In this process the aromatic species is reacted with carbon monoxide in the presence of a catalyst system containing rhodium and halogen components to produce aromatic carboxylic acids or esters.
U.S. Pat. No. 3,769,329 to Paulik et al. differs from the '326 patent in that the identity of the reactant carbonylated to produce a carboxylic acid or ester is, rather than an aromatic alcohol or derivative thereof, a saturated alcohol, a saturated ether, a saturated ester or a saturated halide. This process includes the proviso that if the reactant is other than an alcohol, water is also present.
A process for purifying carboxylic acid streams, particularly acetic acid streams, synthesized in a carbonylation process which involves the reaction of an alcohol or derivative thereof with carbon monoxide, is taught in U.S. Pat. No. 3,772,156 to Johnson et al. In this process hydrogen iodide or an alkyl iodide is employed as a catalyst promotor. The removal of iodine, which is thus present as an impurity, is the inventive feature of the '156 patent. In this process an alkali metal compound, an alkaline earth metal compound or a mixture of the two is added to the acid stream to be purified. Hypophosphorous acid is also added to the acid stream if free iodine is present as the contaminant.
A process for production of monocarboxylic acids is set forth in U.S. Pat. No. 3,813,428 to Paulik et al. In this process an alcohol having the formula R-CH.sub.2 OH, where R is phenyl or hydroxymethyl, is reacted with carbon monoxide in the presence of a catalyst system comprising rhodium or iridium compounds and complexes, together with a halide promoter. This process is applicable also to halide, ester or ether derivatives of the alcohol.
U.S. Pat. No. 3,818,060 to Forster et al. describes a carboxylation process resulting in the synthesis of a carboxylic acid. In this process an ethylenically unsaturated compound is reacted, in the liquid phase, with carbon monoxide and water in the presence of a catalyst system comprising a rhodium or iridium compound, a halide promoter and, as a stabilizer, an organic derivative of pentavalent phosphorous, arsenic, antimony, nitrogen or bismuth.
U.S. Pat. No. 3,845,121 to Eubanks et al. discloses a process for separating carbonylation products from a carbonylation product mixture. In this process an olefin, an alcohol or an ester, a halide or an ether derivative of said alcohol in the liquid phase is reacted with carbon monoxide in the presence of a catalyst system. The catalyst system contains a rhodium or an iridium component and an iodide or bromide component. This liquid product mixture is passed from the reaction zone to a separation zone of substantially lower pressure, without the addition of heat and in the absence of any additional component such as carbon monoxide gas, wherein a portion of the carbonylation product is vaporized and withdrawn from the separation zone.
U.S. Pat. No. 3,887,489 to Fannin et al. sets forth a method of treating a spent catalyst solution which includes a complex reaction product which includes a rhodium component, a halogen component, carbon monoxide and metallic corrosion products. The treatment includes heating the spent solution under agitation at a temperature of about 100.degree. C. to about 190.degree. C. and a pressure sufficient to boil off the carbon monoxide and precipitate the rhodium component. The precipitated rhodium-containing component is redissolved by adding solvent, either water, acetic acid or a mixture thereof, and a halogen component, preferably iodine, under the accompanying thermodynamic conditions of elevated temperature and pressure, said pressure provided by the partial pressure of carbon monoxide gas. This process results in the reformation of the catalyst solution, that is, the redissolving of the rhodium catalyst in the absence of the corrosion products present in the thus processed spent solution.
U.S. Pat. No. 4,433,166 to Singleton et al. teaches a carbonylation process in which an olefin, an alcohol or an ester, halide or ether derivative of the alcohol is reacted with carbon monoxide in the liquid phase. The reaction occurs in the presence of a catalyst system containing a rhodium component and an iodine or bromine component. The liquid reaction mass incorporating these reactants and catalyst system is passed from the reactor to a separator of substantially lower carbon monoxide partial pressure wherein a portion of the carbonylation product, as well as unreacted gaseous reactants such as carbon monoxide, inert gases and unreacted reactants, is vaporized and withdrawn from the separation zone. The invention of the '166 patent lies in the introduction, into the separator, of tin or a tin compound which acts as a stabilizer to prevent rhodium precipitation.
Yet another relevant reference is U.S. Pat. No. 4,690,912 to Paulik et al. which is directed to a catalyst system for the production of carbonylation products by the reaction of a carbonylatable reactant and carbon monoxide. The catalyst system comprises a rhodium-containing catalyst which includes an adduct of a rhodium-containing component source material and carbon monoxide and a separately added iodine-containing promoter component with the proviso that the promoter contains more iodine atoms than the catalyst contains rhodium atoms.